1. Field of the Invention
The present invention relates to a vehicle that is driven by batteries, such as an electric automobile, a forklift, etc.
2. Description of the Related Art
In a vehicle which is driven by a motor using a battery as the power supply, for example, a forklift, increased powering is desired so as to be able to carry a heavy load at higher speed.
The method of increasing the power a forklift is to increase the output of a drive motor. However, in order to increase the output of the drive motor, the diameter of the motor or the length of the motor in the axial direction should be increased. Consequently, there is the problem that the external form of the apparatus that accommodates the motor becomes large. Further, when a motor with high power is newly developed, there arises another problem that the cost of the motor becomes high.
In order to solve such problems, for example, in a model (Japanese Utility Model Laid-Open Publication No. Hei 7-11807) regarding Japanese Utility Model Application No. Hei 5-42904, two direct current motors are connected in series with a battery, an output synthesizing mechanism is connected with the output axes of the two direct current motors, and furthermore the output synthesizing mechanism is engaged with an axle. Thus, an output twice that of a single motor can be obtained.
However, the method of connecting motors in series with a battery is applicable only to a direct current motor, but is not applicable to an AC motor. Further, in order to connect two motors in series, the output voltage of a battery should be high.
For example, in the case of a forklift that is used to carry a load of which the weight is equal to or less than three tons, the output voltage of the battery is generally low, approximately 36V or 48V, and a direct current motor of which the drive voltage is 36V or 48V is used in accordance with the output voltage of the battery.
In the case of realizing a forklift with a heavy load and of which two direct current motors are connected in series, thereby carrying a load of which the weight is equal to or more than four tons, the output voltage of the battery is required to be twice that of the drive voltage of the direct current motor. Therefore, the output voltage should be 72V or 96V. In the case that such a battery and a power circuit of high output voltage are widely used in another apparatus of the forklift, the battery and power circuit can be directly used. However, the battery of the forklift that is used for a load of which the weight is equal to or less than three tons generally has low voltage equal to or less than 50V.
Accordingly, the method of connecting direct current motors in series and obtaining high power requires the usage of a battery of high voltage and the design of a power circuit that is used exclusively for high voltage. Therefore, there is the problem that it takes a long time to develop such a circuit and accordingly the part cost becomes high.
In the case of an AC motor, it is conceivable that a battery of high voltage of which the output voltage is 72V or 96V and two AC motors of 36V or 48V are used. However, in order to use them, the output voltage of the battery is required to be reduced by a transformer, and accordingly the part cost becomes high.
Further, in the case that the battery of high voltage is used, many of the parts other than the motors, such as auxiliary machinery, lamps, etc., of a forklift, have the maximum rating voltage equal to or less than 48V. Therefore, in the case that the battery of high voltage is used, the parts other than motors should be connected with the terminals of 48V of the battery, and there arises the problem that the battery wears asymmetrically.
Furthermore, in the case that the existing battery of low voltage in the forklift is replaced, the output voltage of a new battery should be matched with the existing machine type so that the modification, etc., of the existing charging equipment is not required. Therefore, it is difficult to use the battery of high output voltage of which the machine type is different from that of the existing battery.
The object of the present invention is to obtain high power using a battery and an AC motor of low voltage.
The present invention comprises of a battery, a DC/AC transformer for transforming the output voltage of the battery into AC voltage, a plurality of AC motors connected in parallel to the battery to which voltage is supplied from the DC/AC transformer, and a driving force transmission unit engaging the output axes of the plurality of AC motors and transmitting the driving forces that are obtained from the plurality of AC motors to a wheel.
According to the present invention, a vehicle of high power can be realized at low cost by driving a plurality of AC motors in parallel and synthesizing the driving forces of these AC motors, thereby transmitting the synthesized force to a wheel.
Further, the present invention may be configured by a detecting unit for detecting the output property of at least one AC motor of a plurality of AC motors, and a control unit for controlling the rotation number of the AC motor having the detecting unit and the rotation numbers of other AC motors based on the output property value that is detected by the detecting unit.
Since this configuration enables the rotation numbers of a plurality of AC motors to be controlled based on the rotation number of at least one AC motor, the control of rotation numbers of a plurality of AC motors can be simplified.
For example, the control unit generates a control signal for controlling the output current of the DC/AC transformer based on the rotation number of one AC motor and the desired speed that is indicated by a driver. Thus, the DC/AC transformer that is connected to the AC motor having a detecting unit and the DC/AC transformers that are connected to other AC motors are controlled by the same control signal, and the rotation numbers of a plurality of AC motors can be controlled.
In this way, since control of the rotation numbers of a plurality of AC motors can be controlled by the same control signal, control of the rotation numbers of the AC motors can be simplified.
The driving force transmission unit consists of, for example, the first engaging unit that engages the output axes of the first and second AC motors, and the second engaging unit that transmits the driving force of the first engaging unit to the axle.